Start onions the right way
by ISU Extension and Outreach
March 14, 2020

By Richard Jauron—Iowa State University Extension and Outreach

Learn how to grow delicious onions and green onions.

AMES, Iowa – Onions are an indispensable vegetable in the kitchen. Onions are easy to grow and take up little space. Follow the recommendations of Iowa State University horticulture specialists for successful onion growing. Contact the Hortline at [email protected], or 515-294-3108 with any additional questions.

Which Onion Cultivar Should I Plant?

When considering onions for a home garden, the suggested onion cultivars in Iowa include Blush, Candy, Copra, Patterson, Red Burgermaster, Redwing, Red Zeppelin, Stuttgarter, Walla Walla, and Yellow Sweet Spanish.

An important aspect of onion bulb development is the photoperiod, or day length. Long-day onion cultivars are the best choice for gardeners in Iowa and other northern areas. The amount of onion foliage present at bulb initiation is important, as the onion’s foliage manufactures food for bulb development. Long-day cultivars produce larger bulbs, as they can produce more foliage before bulb initiation occurs.

Onions can be grown from seeds, sets, or transplants. Cost, use, availability, and ease of planting influences the growing method selected. Growing onions from seeds is the least expensive planting method. However, it is also the most difficult.

How Do I Sow Onion Seeds Indoors?

Using a well-drained growing medium, sow onion seeds in flats or plug trays approximately eight to 10 weeks before you intend to plant them outdoors. As soon as the seeds germinate, place the onion seedlings under fluorescent lights. The light fixture should be placed no more than four to six inches above the seedlings. Growing temperatures should be 55-65 degrees Fahrenheit.

Thin the onion seedlings (within two to three weeks of germination) if the plants are crowded. When the seedlings become five inches tall, clip them back to four inches. Harden or acclimate the onion seedlings outdoors for several days before planting. Initially, place the plants in a shady, protected location. Then gradually expose the plants to longer periods of sunlight. Bring the seedlings back indoors if nighttime temperatures are forecast to drop to 32 degrees Fahrenheit or below. Early April to early May is the best time to plant onion seedlings outside.

Plant onion transplants from early April to early May. When planting, place the roots and the lower white portions of the plants below ground level. Space transplants one inch apart when grown for green onions and 2-3 inches apart when grown for mature storage onions.

How Do I Sow Seeds Outdoors?

Onion seeds should be planted as soon as the ground can be worked in spring (late March or early April in southern Iowa, early to mid-April in central Iowa, and mid to late April in northern portions of the state). Cover the seeds with 1/2 to 3/4 of an inch of soil. Thin the planting when the seedlings are two to four inches tall. For large storage onions, plants should be spaced two to three inches apart after thinning.

Before planting sets, separate the bulbs into two size groups — those smaller than a nickel in diameter and those larger than a nickel. The larger sets often bolt (produce a flower stalk) and don’t produce good-sized bulbs. Use the larger sets for green onions. The smaller sets can be allowed to develop into mature onions.

Plant sets from early April to early May. Sets should be planted at a depth of one to one-and-a-half inches. Space sets one inch apart when grown for green onions and two to three inches apart when grown for mature storage onions.

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